1. Field of the Invention
The present invention relates to low NO.sub.x burners for firing fuels such as fuel oil, fuel gas and the like. More particularly, but not by way of limitation, the present invention relates to a burner having an improved register which provides increased regulation of primary and secondary combustion air and decreased formation of NO.sub.x.
2. Discussion
Nitrogen oxides (NO.sub.x) are undesirable by-products of every combustion process. Nitric oxide (NO) and nitrogen dioxide (NO.sub.2) are the primary nitrogen oxides formed, with others such as N.sub.2 O.sub.4, N.sub.2 O and NO.sub.3 produced in only trace quantities. At the temperatures of most combustion applications, the majority of the nitrogen oxides (NO.sub.x) are present as nitric oxide (NO). However, when gases containing nitric oxide (NO) enter the atmosphere, the nitric oxide is converted to nitrogen dioxide (NO.sub.2) as the gas cools. Therefore, NO.sub.x emission calculations usually assume all of the NO.sub.x is in the NO.sub.2 form because this is the form in the atmosphere.
Nitrogen dioxide (NO.sub.2) is a toxic gas that the U.S. Environmental Protection Agency (EPA) has designated as a criteria pollutant because of its adverse effects on human health. Nitrogen oxides (NO.sub.x) emitted from stationary combustion sources contribute to acid rain deposition and to the degradation of air quality by reacting with reactive hydrocarbons to form smog. For this reason, the amount of nitrogen oxides present in gases vented to the atmosphere is heavily regulated by various state and federal agencies and improved combustion techniques are constantly being sought.
NO.sub.x is formed from one of three sources in a combustion process: thermal NO.sub.x, prompt NO.sub.x and fuel bound NO.sub.x. Most NO.sub.x emissions from combustion processes are generated from thermal fixation of nitrogen in the combustion air. The generally accepted mechanism of thermal NO.sub.x formation is described by the Zeldovich equilibrium reactions. EQU N.sub.2 +O.multidot..revreaction.NO+N.multidot. (1) EQU N.multidot.+O.sub.2 .revreaction.NO+O.multidot. (2)
As indicated by the above reactions, thermal NO.sub.x formation requires the dissociation of molecular nitrogen (N.sub.2) and molecular oxygen (O.sub.2). Due to the stability of these molecules, significant dissociation occurs only at high temperatures.
Prompt NO.sub.x is a lesser known type of NO.sub.x formation. The formation of prompt NO.sub.x is proportional to the number of carbon atoms present in the fuel and has a weak temperature dependence and a short lifetime. Prompt NO.sub.x is only significant in fuel rich flames which inherently produce low NO.sub.x levels. Thus, prompt NO.sub.x is not usually a major contributor to overall NO.sub.x emissions.
Fuel bound NO.sub.x is generated from nitrogen compounds present in incinerated waste or in the fuel itself. A significant portion of the fuel or waste nitrogen is converted to NO.sub.x. The rate of conversion is much less than 1/1 however. Yet, as little as 1% conversion produces NO.sub.x concentrations far above regulatory limits. The exact conversion rate is a complex function of stoichiometry, temperature, and the specific nitrogen compound being incinerated; and unfortunately, the detailed mechanisms and kinetics involved in fuel bound NO.sub.x formation are not completely understood.
There have been considerable efforts in the art to reduce (NO.sub.x) in combustion gases so that such gases may be discharged to the atmosphere without harm to the environment. These efforts can be grouped into two categories: "combustion control techniques" and "post combustion control techniques." "Post combustion control techniques" are methods to remove the nitrogen oxides in combustion gases after their formation. The most established of such post combustion control techniques are; SNCR Selective Non-Catalytic Reduction; and SNR Selective Catalytic Reduction.
There are two commercially available SNCR systems. One is commonly referred to as Thermal DeNOx and was originally patented by Exxon, U.S. Pat. No. 3,900,554, issued to Lyon. The other SNCR process is commonly called NOxOUT. Both the Thermal DeNOx and NOxOUT processes involve injection of specific nitrogen bearing compounds, such as ammonia and urea, into the combustion products to reduce NO.sub.x produced during combustion Both reduction reactions occur in a specific temperature range.
Various SCR techniques are known as well. In SCR techniques, as with Thermal DeNOx, ammonia is injected to reduce NO.sub.x. However, in the SCR processes, the ammonia is injected upstream of a catalyst grid and the catalyst changes the optimum temperature range at which NO.sub.x reduction occurs.
Although post-combustion control techniques, such as SNCR and SCR systems, are often employed to reduce NO.sub.x emissions in combustion gases containing NO.sub.x, "combustion control techniques" which prevent the formation of NO.sub.x during the combustion process are more economical methods of meeting NO.sub.x emission requirements. Such combustion control techniques include burner design considerations.
Most modern burner designs rely on the well established technique of recirculation of combustion products back into the flame envelope as a method of NO.sub.x reduction. Many low NO.sub.x burners utilize external recirculation. This technique, called flue gas recirculation (FGR), recycles combustion off-gas into the burner, often after cooling the recirculated flue gas in a heat recovery device. FGR suppresses NO.sub.x formation by lowering the oxygen content in the flame and, more significantly, by lowering the peak flame temperature as a result of the larger mass of gas heated.
Other low NO.sub.x burners achieve similar results using internal recirculation of the products of combustion. Internal recirculation is typically accomplished through a bluff body, swirl vortex, baffle geometry, or toroidal ring. This provides optimum conditions in specific zones of the flame, and the more effectively these conditions are achieved, the more efficient the NO.sub.x reduction.
Still other low NO.sub.x burners function by fuel staging in which a portion of the fuel is mixed with all of the combustion air in the primary combustion zone of the burner. The high level of excess air lowers the peak flame temperature, reducing NO.sub.x formation. Secondary fuel is injected through nozzles located at the perimeter of the burner causing the fuel gas to entrain incinerator gases and mix with the first stage combustion gases. This entrainment of combustion products, as in flue gas recirculation, serves to enhance NO.sub.x reduction from the burner.
The primary combustion control technique, however, is air staging. In this technique, the combustion air is split into two streams. The first portion of combustion air is mixed with the fuel in selected substoichiometric quantities to produce a reducing environment. The second portion of combustion air is injected downstream to complete the combustion. The result is a dual zone combustion process wherein the first zone operates under reducing conditions and the second zone operates under oxidizing conditions.
Many burner design applications operate with combustion air supplied under forced draft conditions. In such a design, a force draft fan supplies air through a set of dampers to a windbox. The dampers help direct the forced draft combustion air toward various regions of the windbox, where air registers distribute the combustion air to the burner as appropriate.
Prior art registers suffer from several drawbacks. Because the forced draft combustion air typically enters the windbox at a selected location on the windbox, prior art registers typically allow the forced draft combustion air to enter the register in an uneven distribution, rather than uniformly around the circumference of the register. In addition, because of the distance between the windbox dampers and the register, zoned registers which are designed to stage combustion air into primary and secondary combustion zones often provide imprecise control over the ratio of primary to secondary combustion air.
Modem low NO.sub.x burner designs generally incorporate one or a combination of the methods and techniques mentioned above to minimize the three factors that contribute to NO.sub.x in combustion systems: (1) flame temperature, (2) residence time of the combustion gases in the high temperature zone and (3) excess oxygen supply. This complex balancing of techniques and variables only serves to intensify the need for greater control over combustion air.
Thus, while there have been considerable efforts to find effective ways to remove or prevent the formation of nitrogen oxides in combustion gases so that the gases can be discharged into the atmosphere without harm to the environment, new and improved devices are constantly being sought which will eliminate the deficiencies of the prior art devices, and which meet the increasingly stringent regulatory requirements placed on combustion gases by federal and state agencies.